mm: mark async iocb read as NOWAIT once some data has been, copied

Once we've copied some data for an iocb that is marked with IOCB_WAITQ, we should no longer attempt to async lock a new page. Instead make sure we return the copied amount, and let the caller retry, instead of returning -EIOCBQUEUED for a new page.

This should only be possible with read-ahead disabled on the below device, and multiple threads racing on the same file. Haven't been able to reproduce on anything else.

---

V3
- Place the 'written' check up top so we catch cases where 'written' is already passed in as non-zero (Kent)

 mm/filemap.c | 8 ++++++++
 1 file changed, 8 insertions(+)

index 1a6beaf69f49..e4101b5bfa82 100644
@@ -2199,6 +2199,14 @@ ssize_t generic_file_buffered_read(struct kiocb *iocb,
 	last_index = (*ppos + iter->count + PAGE_SIZE-1) >> PAGE_SHIFT;
 	offset = *ppos & ~PAGE_MASK;
 
+	/*
+	 * If we've already successfully copied some data, then we
+	 * can no longer safely return -EIOCBQUEUED. Hence mark
+	 * an async read NOWAIT at that point.
+	 */
+	if (written && (iocb->ki_flags & IOCB_WAITQ))
+		iocb->ki_flags |= IOCB_NOWAIT;
+
 	for (;;) {
 		struct page *page;
 		pgoff_t end_index;

2.28.0

-- 
Jens Axboe

It could be useful to elaborate on the (user-visible) failure scenario here a bit, as I don't think it's obvious.

We *wouldn't* return -EIOCBQUEUED, though, would we? We'd do the async path, put the caller on a waitqueue, but then return `written' instead of letting it know.